Fuels such as gasoline, diesel fuel, fuel oil, jet fuel, and kerosene tend to undergo oxidative deterioration during storage whereby solid deposits, gums, or sludge may form in the fuel. Obviously the formation of such materials is detrimental.
Similarly, lubricants and functional fluids such as crankcase lubricating oil, gear oil, electrical insulating oil, spindle oil, automatic transmission fluids, hydraulic fluids, and the like usually require protection against oxidative degradation. In such substrates, premature oxidative deterioration manifests itself in a change in viscosity (usually an increase in viscosity) or an increase in acid number. Such effects are likewise detrimental.